The present invention relates to cellulose materials, i.e. to cellulose or to cellulose derivatives, to liquid-crystal solutions based on such cellulose materials, in particular to spinnable solutions capable of yielding, after coagulation, spun articles such as fibres or films, to these spun articles themselves, and also to processes for obtaining such spun articles.
The invention relates more particularly to an aqueous coagulating agent suitable for coagulating liquid-crystal solutions based on cellulose materials, the use of such a coagulating agent for coagulating such solutions, in particular in a spinning process, and also to a novel cellulose fibre having an unexpected combination of mechanical properties.
It has been known for a long time that the production of liquid-crystal solutions is essential for obtaining fibres having high or very high mechanical properties by spinning, as has been shown in particular by U.S. Pat. No. 3,767,756, which relates to aramid fibres, and U.S. Pat. No. 4,746,694, which relates to aromatic polyester fibres. The spinning of liquid-crystal solutions of cellulose also makes it possible to obtain fibres having high mechanical properties, in particular by what is called the xe2x80x9cdry-jet-wet spinningxe2x80x9d processes, as described, for example, in International Patent Applications PCT/CH85/00065 and PCT/CH95/00206 for liquid-crystal solutions based on cellulose and at least one phosphoric acid.
Patent Application PCT/CH85/00065, published under No. WO85/05115, or its equivalent patents EP-B-179 822 and U.S. Pat. No. 4,839,113, describe the obtaining of spinning solutions based on cellulose formate, by reacting the cellulose with formic acid and phosphoric acid, these solutions being in the liquid-crystal state. These documents also describe the spinning of these solutions using what is called the xe2x80x9cdry-jet-wet spinningxe2x80x9d technique to obtain cellulose formate fibres, as well as cellulose fibres regenerated from these formate fibres.
Patent application PCT/CH95/00206, published under No. WO96/09356, describes a method for dissolving cellulose directly, without formic acid, in a solvent in order to obtain a liquid-crystal solution, this solvent containing more than 85% by weight of at least one phosphoric acid. The fibres obtained after spinning this solution are fibres of non-regenerated cellulose.
Compared with conventional cellulose fibres such as rayon or viscose fibres, or with other conventional non-cellulose fibres, such as nylon or polyester fibres, for example, all spun from optically isotropic liquids, the cellulose fibres described in these two applications WO85/05115 and WO96/09356 are characterised by a far more ordered or oriented structure, owing to the liquid-crystal nature of the spinning solutions from which they have originated. They have very high mechanical properties in extension, in particular toughnesses of the order of 80 to 120 cN/tex, or even more, and initial moduli which may exceed 2500 to 3000 cN/tex.
However, the processes described in the above two applications for obtaining these fibres having very high mechanical properties all have the same disadvantage: the coagulation step is performed in acetone.
Now, acetone is a relatively costly, volatile product, which furthermore has a risk of explosion which requires special safety measures. Such disadvantages are furthermore not peculiar to acetone, but in fact common to numerous organic solvents used in the spinning industry, in particular as coagulating agents.
It was therefore entirely desirable to find an alternative to the use of acetone by replacing it with a coagulating agent which would be more advantageous from an industrial point of view and easier to use, even at the expense of a reduction of certain mechanical properties of the fibres obtained, particularly since the very high mechanical properties described above may be excessive for certain technical applications.
Although it has proved technically possible to replace the acetone with water to coagulate the liquid-crystal solutions described in the two applications WO85/05115 and WO96/09356 mentioned above, experience has shown that the use of water instead of acetone resulted in spinning difficulties and in cellulose fibres having very low toughness compared with those described above, this toughness scarcely ever exceeding 30-35 cN/tex, and reaching at most only 35-40 cN/tex when the fibre being formed is subjected, for example, to particularly high tensile stresses, which furthermore are detrimental to the quality of the product obtained. Such values of 30 to 40 cN/tex are in any case lower than the known toughness values of a conventional fibre of the rayon type (40-50 cN/tex), which nevertheless is obtained from a non-liquid-crystal spinning solution, i.e. one which is optically isotropic.
Thus, for spinning liquid-crystal solutions based on cellulose materials, water has proved to be a coagulating agent which is incapable of producing fibres having satisfactory mechanical properties, in particular a toughness at least equal to that of a conventional rayon fibre, for technical applications, for example for reinforcing rubber articles or tires.
A first aim of the present invention is to propose a novel, water-based coagulating agent which is more advantageous from the industrial point of view than acetone and more effective than water alone, which is capable of producing fibres, the toughness and modulus properties of which are substantially improved compared with those of fibres coagulated simply with water.
The coagulating agent according to the invention, which is capable of coagulating a liquid-crystal solution based on cellulose materials, is characterised by the following features:
it comprises water and at least one additive;
when it is brought into contact with said solution, the kinetics of diffusion of the coagulating agent in the solution and those of precipitation of the cellulose materials under the action of said agent, measured under a microscope in what is called the xe2x80x9ccoagulation testxe2x80x9d for a rate of additive of 20% by weight, are governed by the following relationship:
0.55 less than Kp/Kdxe2x89xa61,
Kdand Kp being respectively the diffusion and precipitation factors (gradients of the respective xe2x80x9cFickxe2x80x9d straight lines), expressed in xcexcm/sxc2xd.
The invention also relates to a process for spinning a liquid-crystal solution based on cellulose materials, for obtaining a spun article, effected using a coagulating agent according to the invention, and also to any spun article obtained by such a process.
Another aim of the invention is to propose a novel cellulose fibre which may be obtained by the process according to the invention; this novel fibre, compared with a conventional rayon fibre, has a toughness at least equal to, if not greater than, a comparable fatigue strength, all combined with a significantly higher initial tensile modulus.
The cellulose fibre of the invention has the following characteristics:
its toughness T is greater than 40 cN/tex;
its initial tensile modulus Im is greater than 1200 cN/tex;
its breaking load degeneration xcex94F after 350 fatigue cycles in what is called the xe2x80x9cbar testxe2x80x9d, at a compression ratio of 3.5% and a tensile stress of 0.25 cN/tex, is less than 30%.
The invention furthermore relates to the following products:
reinforcement assemblies comprising at least one spun article according to the invention, for example, cables, plied yarns, multifilament fibres twisted on themselves, such reinforcement assemblies possibly being, for example, hybrids, composites, i.e. comprising elements of different natures, possibly not in accordance with the invention;
articles reinforced by at least one spun article and/or an assembly according to the invention, these articles being, for example, articles made of rubber or of plastics material(s), for example plies, belts, tubes or tires, in particular tire carcass reinforcements.
The invention and its advantages will be readily understood in the light of the following description and non-limiting examples, as well as FIGS. 1 and 2 appended to the description.